This invention relates generally to a fluorescent lamp ballast and, more particularly, to a starting capacitor disconnect scheme for a fluorescent lamp.
A fluorescent lamp ballast for lighting serially connected fluorescent lamps typically employs one or more capacitors (commonly referred to as starting capacitors) for starting purposes. Each starting capacitor is connected across at least one of the serially connected lamps. The voltage produced across the output of the ballast is insufficient to ignite all serially connected lamps at the same time. The starting capacitor acts as a shunt across the one or more lamps that the capacitor is connected in parallel with. A voltage sufficient for ignition of the unshunted lamp then can be applied thereto. Once the unshunted lamp has been ignited, the one or more shunted lamps are then ignited because the voltage produced by the ballast now is sufficient to ignite these one or more shunted lamps.
Current spikes produced by the starting capacitor are considered by lamp manufacturers to adversely affect lamp life through excessive sputtering of emissive electrode material onto the inner walls of the lamp. The sputtered electrode material which covers the inner lamp wall reduces the lumen output. The starting capacitor also redirects a portion of the available current away from the shunted lamps resulting in less light by the latter.
Accordingly, it is desirable to provide a fluorescent ballast having improved starting properties so as to increase lamp life and maintain nominally rated lumen output for a longer period of time. The fluorescent ballast should substantially eliminate both current spikes produced by the one or more starting capacitors and the diversion of current by the one or more starting capacitors away from the one or more lamps once all lamps have been ignited.